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Conservation activists are highlighting 350 species imperiled by global warming as part of Saturday's International Day of Climate Action

The Center for Biological Diversity created an interactive Web site today listing the 350 species and how they are threatened by climate change.

Climate scientists say 350 parts per million of carbon dioxide in the atmosphere is the upper limit for an environmentally healthy Earth. The count is now at about 387 parts per million, and Saturday's actions are directed at world leaders to take aggressive, immediate steps to limit carbon emissions.

"The Arctic is already melting, sea level is already rising and polar bears are already dying," said Rose Braz, of the Center for Biological Diversity. "We need quick action to reduce atmospheric carbon dioxide to 350 parts per million, and the current legislative proposals won't get us there."

To meet the goal, the United States would need to cut its greenhouse gas emissions to 40 percent or more below 1990 levels by 2020, activists say. World leaders will meet in December in Copenhagen to hammer out a global policy on carbon emissions.

"If we fail to reduce atmospheric carbon dioxide to 350 parts per million or below, many thousands of species, including our own human race, face a perilous future," Braz said.

Among the 350 species are:

Corals, which are at high risk of extinction worldwide because of global warming and ocean acidification.

Sea otters on the West Coast, which will have difficulty finding their main prey of invertebrates as increasingly corrosive waters make it harder to form shells.

The Pacific walrus, which is being considered for protection under the Endangered Species Act. Its home of Arctic sea ice is shrinking, and young walruses have been trampled to death as larger numbers move closer to shore.

Polar bears, two-thirds of which will disappear by mid-century as sea-ice habitat melts, U.S. government scientists predict. The U.S. Fish and Wildlife Service listed the polar bear as threatened under the Endangered Species Act last year because of climate change.